Method for the pyrolytic deposition of silicon carbide



United States Patent 3,416,951 METHOD FOR THE PYROLYTIC DEPOSITION OF SILICON CARBIDE Ralph L. Hough, Springfield, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force No Drawing. Filed July 28, 1965, Ser. No. 475,609 6 Claims. (Cl. 117-106) ABSTRACT -.OF THE DISCLOSURE A pyrolytic method for uniformly depositing SiC on a metal substrate, such as a W filament, comprising the steps of (1) bubbling a carrier gas, such as H, through a selected liquid, such as methyltrimethoxysilane, which is the source of the SiC, so as to vaporize said silane in part, and (2) contacting said substrate heated at a temperature from 1200-1800" C. under a pressure of one atmosphere and less with said vapor.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The present invention relates to a new method for the manufacture of pyrolytic deposition of silicon carbide.

In response to the ever-increasing number of applications wherein high temperature environments are encountered, particularly in the arts of high speed aircraft, aerospace vehicles and the rocket engines employed to power the same, increasing attention has been focused upon the manufacture and manipulation of a variety of ablative and refractory materials. Of particular interest in these areas have been the carbides and especially silicon carbide; and a prominent method for the formation of these has been the pyrolytic deposition thereof upon metal substrates which are pre-designed according to the ultimate use intended for the refractory coating. In many cases the substrate is in the form of a filament which is continuous or of substantial length and is composed of a finely drawn ductile metal such as tungsten. In the usual practice, the filament is fed continuously through a suitably constructed pyrolytic deposition chamber which is filled with volatilized precursory materials so chosen that the vapor within the chamber will contain the elements which are desired to be plated out as a coating upon the substrate. The substrate, while exposed to this gaseous atmosphere, is brought to sufiiciently high temperatures that a gas-phase plating reaction between the deposition gas and the substrate surface takes place. The resultant filamentous product is then usable as a continuously wound reinforcement within various ablative plastic binders or matrices such as phenolic resins in the manufacture of high temperature components such as the throats and exhaust nozzles of rocket engines, the leading edges of high speed aircraft and the nose cones of atmospheric escape and re-entry vehicles.

Because these filaments are generally extremely small, being on the order of from 0.2 to 1.0 mils in diameter and the pyrolytic coating about the metal core or substrate is of proportionately small dimensions, the quality of the pyrolytic material throughout its entire thickness; i.e., from the first crystallites deposited upon the substrate to the outer surface of the finished coating, is of critical importance. The film deposited immediately upon the substrate at the very beginning of the deposition operation is perhaps even more critical however in that the crystallites first deposited represent growth sites from and upon which the balance of the coating will be built up. Any lack of uniformity at the inception of the deposition process will accordingly result in a haphazard growth of the deposited 3,416,951 Patented Dec. 17, 1968 material and the presence throughout its thickness 01 large grains or nodules representing serious weaknesses ir the coating which will not remain adhered to the substrate and will destroy the structural and refractory characteristics of the reinforcement. Moreover, the graininess 01 the coating or the presence of nodules therein or on the surface thereof interferes with the mechanical manipulation of the filaments to obtain optimum refractory and reinforcing effects in the composite material of which they are a part. While the prior art has provided several methods for the formation of silicon carbide including those involving pyrolytic deposition, it has not met the critical demands for uniform quality and crystallite development which will result in a dense deposit of the carbide to the degree of consistency and uniformity which maximum reinforcement and heat resistance generally require.

It is accordingly an object of this invention to provide an improved method for the manufacture of silicon carbide by pyrolytic deposition means.

Yet another object of the invention is to provide such a method which will result in the formation of a uniformly dense carbide coating on a filamentous substrate.

Yet another object of the present invention is to provide such a method which will result in a coating which is free of graininess and nodules therein or upon the surface thereof.

Yet another object of the invention is to provide a method for the pyrolytic deposition of silicon carbide wherein the initial deposition of the carbide upon the substrate surface will be uniform in character and will not lead to nodular development from irregular growth sites.

To achieve these and other objects and advantages which will appear from a reading of the following disclosure, the present invention teaches the use of a novel series of compositions as sources of the silicon and carbon to be involved in the vapor-phase plating reaction resulting in the carbide formation. More specifically the invention teaches the use of a gas or gaseous mixture including silicon and carbon atoms to form the carbide and carbonyl functional groups which have been found to play an important part in achieving the above enumerated objects. Though the reasons for the improvement are obscure and not understood, experimentation has revealed that the presence of the carbonyl group in the gas or gaseous mixtures according to this invention leads to the formation of a dense uniform coating upon a substrate surface so that the deposition of such gas upon a filamentous substrate results in a smooth, continuous, high-quality coating capable of achieving the highest reinforcing and refractory characteristics thus far available in such silicon carbide filaments. One such deposition gas comprises a carbonyl functional group-containing silane as the source for the carbon and silicon atoms as well as the carbonyl group. Another such gas according to this invention comprises an admixture of a silicon halide as the source of the silicon atoms and of a carbonyl group-containing hydrocarbon such as a ketone as the source of both the carbon atoms and the carbonyl group.

In one preferred method for practicing the invention, a carrier gas such as hydrogen is blubbed through containers of silicon tetrachloride and a ketone such as acetone which are liquid at room temperatures. In conventional bubbling apparatus wherein the gas passes through the liquid, it volatilizes a portion thereof and carries it as a vapor into the deposition chamber by suitable conduits, connections and fittings. The rate of flow of the carrier gas which, in certain instances, may be an inert gas such as argon, helium, neon, krypton or the active gases such as hydrogen, is so adjusted relative to the temperatures of the precursory liquids and other conditions influencing the rate of evaporation that the deposition gas within the deposition chamber will comprise a sufiicient volume of he hydrocarbon gas as compared with the silicon halide ;as that the ratio of silicon atoms in the total gas will be tpproximately three times the number of the carbon atoms upplied by the hydrocarbon material excluding the carbon \toms therein which are part of the carbonyl functional group. When this deposition gas contacts the surface of he substrate which is preferably heated to a pyrolytic leposition temperature of from 1200 to 1800 degrees Ientigrade under reduced pressures within the chamber of rom .5 to 5 millimeters of mercury although atmospheric )ressure can be used. The reduced pressures serve prinarily to remove volatile secondary pyrolysis-by products. k plating reaction takes place according to the following :quation:

2SiCl (CH CO+H 2SiC+ SHCl-l-CO While in this preferred example, acetone was used as the ource of the carbon atoms and the carbonyl functional group, any carbon containing ketone with at least one :arbonyl functional group in the molecule will provide he beneficial results of the present invention.

In another method for practicing the within invention, I. silane containing a carbonyl functional group such as l. methoxysilane may be used as the source for both the :arbon and silicon atoms as well as for the carbonyl funcional group. By way of specific example, methyl trimeth- Ixysilane such as Z-6070 Silane (manufactured and sold inder that designation by Dow Corning Corporation of viidland, Michigan), which is a liquid at room temperaure, may be volatilized or vaporized by the passage of iydrogen or an inert carrier gas through a bath of the iquid and carried into the deposition chamber where it :trikes a heated substrate. Other alkyl methoxysilanes vhich may be employed include dimethyldimethoxysilane,

ll'l important preferred series of silanes being those wheren the carbon to silicon ratio of the atoms within the nolecule is approximately one to one excluding again the :arbon atoms which are a part of the carbonyl functional group. Under the influence of the heat supplied by the .ubstrate within the deposition chamber, the following 'eaction takes place to form silicon carbide in the case of l deposition gas composed of the methyl trimethoxysilane:

CH3Sl(O CHzDa T 3CHsOHSiC 01-13011 A further pyrolysis As in the case of the ketone-silicon halide gaseous mixture, the deposition of the silicon carbide from the methoxysilane should also take place within the chamber under reduced pressure of on the order of from .5 to 5.0 millimeters of mercury although atmospheric pressures can be used.

While the within invention has been described in considerable detail with regard to certain specific examples and embodiments thereof, it is to be understood that the foregoing particularization has been for the purpose of illustration only and does not limit the scope of the invention as it is defined in the subjoined claims.

I claim:

1. A pyrolytic method for uniformly depositing silicon carbide on a metal substrate comprising the steps of (1) bubbling a carrier gas selected from the group consisting of H, A, He, Ne and Kr through a liquid selected from the group consisting of (a) SiCL, and (CH CO and (b) an alkylmethoxysilane whereby a portion of the liquid is volatilized to form a vapor and (2) contacting said substrate heated at a temperature from 1200 to 1800" C. with said vapor under a pressure of one atmosphere and less.

2. A method according to claim 1 wherein said alkylmethoxysilane is dimethyldimethoxysilane.

3. A method according to claim 1 wherein the metal substrate is a filament.

4. A method according to claim 3 wherein the filament is tungsten.

5 A method according to claim 1 wherein said pressure is on the order of .5 to 5.0 millimeters of mercury.

6. A method according to claim 1 wherein said alkyl methoxysilane is methyltrimethoxysilane.

References Cited UNITED STATES PATENTS 3,011,912 12/1961 Gareis et al. 3,065,050 11/1962 Baumert. 3,074,817 1/1963 Gentner. 3,099,534 7/ 1963 Schweickert et al. 3,157,541 11/1964 Heywang et al.

RALPH S. KENDALL, Primary Examiner.

A. GOLIAN, Assistant Examiner. 

